The present invention relates to a spherical semiconductor device and the manufacture method for the same. The spherical semiconductor device can be applied to various uses as a photo micro solar cell, detecting element, light emitting element or as a photocatalytic element. The spherical semiconductor device comprises a semiconductor thin film layer on the surface of a small spherical core made of a semiconductor material, insulating material or metallic material, and has at least one pair of electrodes. The present invention also relates to a spherical semiconductor device material.
Various solar cells in which a semiconductor converts solar energy into electrical energy are widely used. With these solar cells, the semiconductor substrate is planar. The light-receiving surface as well as the pn-junction which is formed in its interior are also largely formed as a flat surface. The electrodes as a mechanical support are provided on the substrate side. As a result, the light-receiving surface is limited to the front surface, and photoelectric conversion from the backside is not possible. Furthermore, as the angle of incidence of the light becomes larger, light reflection increases, and the photoelectric conversion efficiency is reduced.
The semiconductor photocatalyst generates a photovoltage when it receives solar light and causes an electrochemical reaction by this photovoltage. Metal oxide semiconductors such as titanium dioxide (TiO.sub.2), strontium titanate (SrTiO.sub.3), or the like have been utilized as the semiconductor photocatalyst. Photocatalysts in which a metal such as platinum or the like is supported on a powder of metal oxide semiconductor have been researched. Electrodes formed by a thin layer of titanium dioxide on one side of titanium plate have also been researched. Because metal oxide semiconductors such as titanium dioxide or the like have a large energy band gap, electrolysis of water is possible, and they do not dissolve in an electrolytic solution. However, titanium dioxide does not function as a photocatalyst when the light spectrum has a wavelength of approximately 410 nm or greater. As a result, the photoelectric conversion efficiency with respect to sunlight is low.
In U.S. Pat. No. 4,021,323, there is described a technology, wherein: small amounts of molten silicon melt are sprayed from a small nozzle which is placed on the upper end of a shot tower; silicon melt is allowed to free fall, and spherical crystals of silicon are created. However with this technology, impurities from the nozzle may become dissolved into the molten silicon. Furthermore, because there is a volume increase when molten silicon solidifies, and because solidification begins from the surface, the part which solidifies last will protrude towards the surface of the spherical crystal, and a protruding area is formed. A truly spherical sphere crystal can not be formed. However, with the drop tube type experimental apparatus of NASA, because it is equipped with an electromagnetic levitation heating equipment, the material is allowed to melt without crucible and free fall.
In this U.S. Patent, a pn junction which can conduct photoelectric conversion is formed on the spherical crystal of silicon. There is also disclosed a solar cell array where a plurality of these spherical crystals (micro photocells) are lined up and connected to a common metal electrode film. There is also disclosed a photochemical energy conversion device wherein: these solar cell arrays are submerged in electrolyte solution; and electrolysis of a solution of hydroiodic acid and hydrobromic acid proceeds by the photovoltage provided by the photoelectric conversion of sunlight.
However, because each of the spherical crystals are attached to the metal electrode film which is the common electrode, incident light can only be received from the front surface side. Because a plurality of micro photocells share the metal electrode film, each individual micro photocells can not be used as independent solar cell elements. As a result, the micro photocells can not be dispersed in the electrolyte solution, their installation positions can not be changed, nor can they be recovered and reused or washed. The limitations in its use as a semiconductor photocatalyst are extremely large. In addition, in this USP, there is no disclosure regarding the use of semiconductors with photocatalytic function as electrodes for the micro photocells, nor is there disclosure regarding the use of semiconductors which have photocatalytic function and which are selected by considering the reaction activity or reaction selectivity. In the art of this US patent, each of the microphotocells described above do not have their own pair of electrodes. A single or a plurality of spherical semiconductor elements having a pn junction can not be incorporated into a semiconductor device in such a way that they are independent cells or elements. Because the mode of electrical connection of the plurality of spherical semiconductor elements is fixed, it lacks in generalizability and is not practical.
In a previous international patent application (PCT/JP 96/02948), the present inventors have proposed a new spherical semiconductor device which can be used variously as a light detecting element (photoelectric conversion element), light emitting element (electrophoto conversion element), or photocatalytic element. This spherical semiconductor device is basically a spherical crystal of semiconductor with a pn junction (or a MIS configuration, Schottky barrier) and a pair of electrodes. For the photocatalytic device, an electrode coating of a oxide semiconductor which has a photocatalytic function is formed on one electrode. In the previous international patent application, the following devices are proposed: a device where the spherical semiconductor device is used alone; a device where a plurality of spherical semiconductor devices are connected in series; a device where a plurality of spherical semiconductor devices are placed in a matrix; an electrolysis device where a plurality of spherical semiconductor devices are scattered in an electrolyte solution.
In the present invention, we propose a spherical semiconductor device and the manufacturing method for the same in which the spherical semiconductor device proposed in the previous international patent application is improved.